Pulse propagation analysis has long been proposed as a tool for nonintrusively detecting anomalies in elongate, conductive members such as buried cables and the like. In U.S. Pat. No. 4,970,467, the present applicant proposed propagating two electrical pulses along a conductor such that they intersect at a predetermined location along the conductor. By analyzing one of these pulses after they have passed through the predetermined location, the applicant found that the presence or absence of an anomaly at the predetermined location could be predicted.
The basic principle disclosed in the '467 patent was also used in U.S. Pat. Nos. 5,189,374, 5,243,294, and 5,270,661, all also issued to the present applicant. The '374 patent applied the basic principles described in the '467 patent in the context of a conductive member, such as a well casing, where one end of the member is more accessible than the other end. The '294 patent teaches that the propagation delay for the length of the conductive member under test can be used to set the timing of the pulses such that the pulses intersect at a plurality of desired locations along the portion of the conductive member under test. The '661 patent teaches applying a DC electrical current to the member to provide an electrical potential. Changes in the electrical potential are observed and correlated with analysis of pulses that have intersected along the member.
While the basic principles described in the foregoing patents are generally applicable to any conductive member, the inventions claimed in those patents were, for illustration purposes, disclosed in the context of a buried pipe. In contrast, the present invention is of particular significance when used in the context of detecting surface anomalies, such as corrosion, on insulated pipe located above ground. Accordingly, that application will be described in detail herein. The present invention may, however, have broader application to detecting other anomalies on elongate, conductive members. The scope of the present invention should thus be determined not based on the following detailed description but instead on the claims appended hereto.
In manufacturing facilities such as oil refineries and the like, miles of pipe are used to carry fluids being processed or used in the refining process. The failure of such pipes can cause extensive damage, and these pipes are often routinely inspected to avoid pipe failures and the damage resulting therefrom. For a variety of reasons, such as energy conservation and worker safety, many of these pipes are wrapped with insulation.
For any given pipe, the entire interior surface of the pipe is subject to essentially the same conditions. The interior of the pipe thus can be satisfactorily tested by sampling analysis of the interior surface at discrete points and using statistical analysis to draw inferences regarding the condition of the pipe at locations between the sampled points.
The exterior surface of insulated pipe cannot be adequately tested using sampling and statistical inferences, however, because one cannot assume that the entire exterior surface of insulated pipe is subject to the same conditions. To the contrary, anomalies on the exterior surface of a pipe tend to be localized and caused by factors specific to that location.
Accordingly, to inspect the exterior surface of insulated pipes, the insulation must be removed and the exterior surface visually inspected. The process of removing and reinstalling this insulation is very expensive. In this context a buried pipe will be considered one form of insulated pipe, as buried pipe cannot be visually inspected without expensive excavation. Accordingly, the need exists for a method of testing the condition of the exterior surface of an insulated pipe by means other than visual inspection after the insulation has been removed.